Amsteel 101

Everything you always wanted to know about Amsteel but were afraid to ask.

Is Amsteel the same thing as Amsteel Blue?
They are not the same thing, but are often confused. As an example, the average strength of 1/4” Amsteel is 7400 pounds, while Amsteel Blue is 8600 pounds. Samson’s Amsteel is made from Dyneema SK60. Amsteel Blue is made from Dyneema SK75. What is being sold by cottage vendors today is usually Amsteel Blue, even if they don’t always call it that.

Does Amsteel Blue have 8 strands or 12 strands?
It depends. The 7/64 commonly used for whoopie slings, has only eight strands, but diameters beyond that have twelve. The Samson documentation invariably lists Amsteel Blue as a (class 2)12-strand product, but that documentation often does not include the 7/64 size because it is too small for the marine and industrial use that Samson caters to.

Class I ropes are manufactured from polyolefin, nylon and or polyester fiber. Class II ropes (including Amsteel) are manufactured from high-modulus fiber, such as Dyneema.

What is high modulus?
Amsteel is defined by Samson as a “high-modulus polyethylene” rope. The high-modulus part means that Amsteel has “low elasticity elongation” or in other words, it doesn’t stretch much. However, this also means that this type of rope doesn’t like to be shock-loaded, so ease into the hammock! Safe working loads do not apply to shock loading.

What is the safe working load (SWL) of Amsteel 7/64?
Working load (WL) is the load that a rope is subjected to during normal use. WLs are based on a percentage of the breaking strength (aka tensile strength) of new and unused rope. They are calculated by dividing the rope’s breaking strength (BS) by a safety factor (SF). The breaking strength of Amsteel Blue 7/64 is 1600 pounds. Samson’ recommends that maximum workload should be 1/5th, or 20% of the quoted breaking strength (safety factor = 5), and recommends a higher safety factor for uses that involve “life or limb.” The Cordage Institute’s recommends safety factors of 5 to 12 for non-critical uses, and 15 for life lines.

Here is the weight that a typical 7/64 Amsteel Blue whoopie sling is rated to support, using the minimum safety factor recommended by Samson:

SWL = BS / SF = 1600 / 5 = 320 lbs

Note: I had originally posted a calculation based on plain Amsteel, which gave rise to some of the responses below. Don’t get confused.

What size bury for Amsteel 7/64?
Samson specifies that a fid is equal to the diameter of the rope x 21, and that an effective bury for Class 2 rope should be three and a half fids. The diameter of Amsteel 7/64 is 0.11 inches, so for this rope, one fid is equal to 2.31 inches. So the recommended bury would be about 8 inches.

Bury = fid x 3.5 = 2.31 x 3.5 = 8.01 inches

Note that the length of the bury is dependent on the diameter of the rope. Counter-intuitively, the smaller the diameter of the rope, the shorter the bury needs to be. For example, a whoopie made with 1.75 mm Zing-it would only need five inches of bury (if following Samson specs), while one made with 1/8 inch Amsteel would need nine inches.

To stitch or not to stitch?
The eye of the whoopie sling is usually made with a locked Brummell, which does not require stitching. The eye made in a variation of the “utility constrictor rope” (UCR), does require lock stitches (I like UCRs better than whoopies).

Why use splicing instead of knots?
Knots degrade the strength of the rope up to fifty percent. A good splice should not reduce rope strength by more than ten percent.